Modern utilizations of substances in electronics, such as radio transmitters and high frequency microphones, often require single crystals with controlled purity and structural perfection. Previous techniques include growing the crystal from a melt, from a solution, or from a silica gel. However, the methods which require high temperatures, the melt and solution techniques, are likely to produce less perfect crystals, due to the higher frequency of dislocations produced by thermal vibration, the increased solubility of impurities, and the greater number of point imperfections caused by thermal stress during cooling. Accordingly, the use of silica gels as a crystal growth medium has developed into a common method for growing single crystals with structural perfection and controlled purity. The near convectionless environment of silica gel suppresses nucleation, thereby reducing the competitive nature of crystal growth. This competition limits the size and perfection of the crystal: and it is obviously desirable to suppress nucleation until, ideally, only one crystal grows in a predetermined location. However, a silica gel is not a completely convectionless environment like outer space: but it is the closest known environment to that of outer space that can be created on earth.
Alkali metal silicate solutions for the preparation of silica gels are known in the art. In general, a silica gel is prepared by mixing equal volumes of an alkali silica solution, preferably sodium silicate, and a dilute acid and allowing the mixture to gel at a temperature between 20.degree. C. and 45.degree. C. The use of this method produces a cloudy, translucent gel. However, crystals which are grown in gels can be more easily observed if the gel is transparent.
Gels can also be formed by dissolving an aluminum salt in a quantity of acid before mixing with waterglass or sodium metasilicate. The use of this method produces a transparent gel within a few minutes. These gels have complicated ionic adsorption properties, and when crystals are grown in them, there is a tendency for contamination of the crystals with aluminum. For this reason, they are ordinarily not used as a crystal growth medium, despite their otherwise attractive transparency.
Accordingly, it is an object of the present invention to produce silica gels which can be used as a crystal growth medium to simulate the convectionless environment of space.
It is another object of the present invention to produce silica gels which will not contaminate crystals that are grown in the gels.
Another object of the present invention is to produce silica gels which are transparent and allow the observation of crystals grown therein during the growth period.